I feel like we're often way too quick to judge something as self-insert, plot device etc. We should be delving a bit deeper, I think.
We're at the point, where common folk more often IS literate and where books are there for the taking, even if not easily obtainable. Gal encounters a book about controversial topic, which on the other hand does not render the most popular worldview invalid (doesn't reject God or catholicism for example). Moreover, it's written by a commoner (she doesn't know, sure) with language understandable to commoners.
I don't think the idea of earning money from publishing such a book is something unbelievable, especially followed by "hey, it might've already been capitalized on, damn". Also - she didn't have God to turn to for erasing her doubts and pains. Books and gaining knowledge and money is what she had, pointed in that direction by her loving uncle xD